FormattingFunctions to format numbers, strings, dates, etc.
Hook implementations for user-created content typesFunctions that implement hooks for user-created content types.
HooksAllow modules to interact with the Drupal core.
HTTP handlingFunctions to properly handle HTTP responses.
Image toolkitsFunctions for image file manipulations.
Input validationFunctions to validate user input.
Language addition APIAdd a language.
Language administration interfaceAdministration interface for languages.
Language negotiation optionsFunctions for language negotiation.
List of predefined languagesAPI to provide a list of predefined languages.
Locking mechanismsFunctions to coordinate long-running operations across requests.
Logging severity levelsLogging severity levels as defined in RFC 3164.
Menu context typesFlags for use in the "context" attribute of menu router items.
Menu flagsFlags for use in the "type" attribute of menu items.
Menu item typesDefinitions for various menu item types.
Menu status codesStatus codes for menu callbacks.
Menu systemDefine the navigation menus, and route page requests to code based on URLs.
Menu tree parametersParameters for a menu tree.
Node access rightsThe node access system determines who can do what to which nodes.
Node API HooksFunctions to define and modify content types.
Pager piecesTheme functions for customizing pager elements.
PHP wrapper functionsFunctions that are wrappers or custom implementations of PHP functions.
Private update status cache systemFunctions to manage the update status cache.
Queue operationsQueue items to allow later processing.
RDF Mapping APIFunctions to describe entities and bundles in RDF.
Sanitization functionsFunctions to sanitize values.
Schema APIAPI to handle database schemas.
Search interfaceThe Drupal search interface manages a global search mechanism.
Standard filtersFilters implemented by the filter.module.
Taxonomy indexingFunctions to maintain taxonomy indexing.
Translation administration screensScreens for translation administration.
Translation editing/deletion interfaceEdit and delete translation strings.
Translation import/export API.Functions to import and export translations.
Translation search APIFunctions to search in translation files.
Update manager: file managementUpdate manager file management functions.
Update manager: installUpdate manager for installing new code.
Update manager: updateUpdate manager for updating existing code.
Update versions of API functionsFunctions similar to normal API function but not firing hooks.
Updates from 6.x to 7.xUpdate functions from 6.x to 7.x.